This invention relates to cooling systems and, more particularly, to cooling the tip perimeter of a turbomachinery rotor blade.
Turbomachinery rotor blades of certain varieties operate in extremely high temperature environments. In order to maintain the blades in operable condition, means are provided for routing cooling fluid (usually air) to the blades for reducing the high surface temperatures. One area which is particularly troublesome in this regard is the blade tip, the radial extremity of the blade.
One characteristic of the blade tip which makes it difficult to cool is the fact that it is disposed in proximity with a circumscribing shroud. The shroud serves to define a flow path for the operating fluid of the turbomachine, and the proximity between the shroud and the blade tip is the result of attempts to improve engine efficiency by minimizing leakage of operating fluid past the blade tips. In order to cool the blade tip a recessed cap has been provided in the prior art which combines with the side walls and shroud to form a tip space within which cooling air is passed from a blade internal cavity.
In addition to defining a cavity for cooling the tip area, the radial extremities of the side walls tend to form a labyrinth seal for inhibiting the leakage of the operating fluid (often in excess of 2000.degree. F.) between the blade tip and the shroud from the blade airfoil pressure surface to the suction surface, leakage which reduces the aerodynamic efficiency of the turbine. It is well understood that maximum engine efficiency requires minimum cooling air usage which, in turn, demands that cooling air application be as efficient as possible. In furtherance of this aim and as previously mentioned, the tip space of the prior art is generally cooled by cooling air passed from an internal blade cavity to the tip space by means of at least one aperture in the cap. However, as the temperature of the working fluid steadily increases in advanced technology turbomachinery, the extreme tip of the blade, comprising the radial extremities of the side walls extending beyond the tip cap, is extremely difficult to cool due, in part, to the need for a generous allowance of rub material in the event that the rotating blade contacts the proximate circumscribing stationary shroud. In other words, the tip cap is recessed to remove it from close proximity with the circumscribing shroud to avoid rubbing contact therebetween. This requires a clearance gap of from approximately 0.1 to 0.15 inch. Thus, the difficulty in cooling. Cooling of these extremities could be accomplished in the manner of the prior art by dumping larger amounts of air into the tip space, but the amount of air required to provide effective cooling thereof would be undesirable from a performance cycle standpoint. Furthermore, a solution comprising a substitution of materials at the extreme tip of the blade to better withstand the high temperatures is not workable at this time since no known reasonably priced metallic material or means for reliable attachment can withstand the temperatures of current advanced technology engines without supplemental cooling.
The present invention provides a solution to these problems with the prior art by the provision of a multiplicity of generally radial passages formed within the radial extremities of the side walls communicating with the blade internal coolant cavity to provide cooling thereof.